The present invention relates to a display device for holographically reconstructing a three-dimensional scene, said display device having an improved reconstruction quality thanks to the reduction of speckle patterns.
This invention can for example be applied in holographic display devices which are used to generate, store and reconstruct holograms of the three-dimensional scene in real-time or near-real-time processes with the help of coherent laser light. The reconstruction of the scene in such a display device is visible through a visibility region, which is also referred to as observer window, in a reconstruction space.
The method for reconstructing a scene, where the reconstruction is visible through an observer window, and examples for the computation and encoding of the hologram of the scene have been described in earlier documents filed by the applicant, for example in (1) EP 1 563 346 A2 and (2) DE 10 2004 063 838 A1.
Further, those documents describe a holographic display device in which the above-mentioned method for the reconstruction of a hologram is implemented. The reconstruction method will be briefly explained below:
For the holographic reconstruction, a three-dimensional scene is sliced by software means into section layers, each of which comprising a multitude of object points of that scene. The object points characterise both the section layer and, as the sum of all layers, the three-dimensional scene.
A computer-generated hologram (CGH) is computed based on the object points as a two-dimensional arrangement of generally complex values, which are represented on a light modulator means. The light modulator means comprises regularly arranged, controllable elements for the modulation of the wave fronts of the incident coherent light with the complex values of the encoded scene. The reconstruction of the scene is generated in a reconstruction space with the help of the coherent light and a reconstruction means, which is controlled by system controller means. The wave fronts of the reconstructed object points are coherently superimposed in the observer window. An observer sees from an eye position in that observer window the resultant reconstruction of the scene in the reconstruction space, which stretches between the observer window and a modulator means or screen.
According to a modified version of this method, a reconstruction of the scene can also be generated by computing individual CGHs from the individual object points, and by encoding separate regions on the light modulator means with those sub-holograms. The phase distribution of the complex values in the region of the sub-hologram roughly corresponds with a holographically encoded lens function, which reconstructs that single object point in its focal point. The focal length of such a lens depends on the axial distance of the object point from the light modulator means or screen.
The absolute value of the complex values, i.e. the amplitude, is about constant across the entire sub-hologram, and its magnitude depends on the axial distance of the object point from the screen or light modulator means, and on the brightness of the object point. As coherent light passes through the light modulator means, the complex transparency values which are encoded there modify the amplitude and/or phase of the light. The object point is reconstructed by the modulated light. Outside the sub-hologram, this object point has the value zero on the light modulator means, i.e. it is only represented by the sub-hologram. The total encoded hologram of the scene is generated by adding the complex values of the individual sub-holograms.
According to a simplified version of the method, object points are for example combined according to certain criteria so to form object point groups, where each group is represented by one CGH in a sequential process. Their wave fronts are in that case superimposed incoherently in the observer window and generate a resultant reconstruction of the scene in the reconstruction space.
This is described with several computation and representation options in hitherto unpublished documents filed by the applicant, e.g. in DE 10 2006 062 377 and DE 10 2007 023 738.
For watching the reconstruction of the three-dimensional scene, the observer can either look at a light modulator means on which a hologram of the scene is directly encoded, and which serves as a screen. This is referred to as a direct-view arrangement. Alternatively, the observer can look at a screen onto which an image of the hologram values encoded on the light modulator means is projected. This is referred to as a projector arrangement.
The eye positions of observers are detected by a position finder in a known manner, said position finder being linked by software means with a storage means and a computing unit, and with a system controller means. The storage means also hosts the information of the object points which are necessary for computing the CGH in data records in the form of a look-up table.
The size of the observer window in front of a display means is defined; it is typically as large as an eye pupil. Seen from the wave-optical point of view, an observer window is formed either by a direct or inverse Fourier transform or Fresnel transform of a hologram encoded on a light modulator means, or by the image of a wave front encoded on a light modulator means in a plane of a reconstruction space. The observer window only comprises a single diffraction order of a periodic reconstruction of the scene. The plane may be a focal plane of a focussing means, or the image plane of a light source. The hologram or the wave front are computed from the scene such that, within the one diffraction order which serves as the visibility region, cross-talking between the observer eyes is prevented, which would typically occur in reconstructions when using light modulators. In conjunction with an arrangement or a method for suppressing higher diffraction orders, scenes can be consecutively presented in a multiplex process to a left and to a right eye of an observer without any cross-talking. Moreover, a multiplex process with the aim to serve multiple persons only then becomes possible.
The pixels of spatial light modulators, such as LCD, LCoS etc., which modulate the phase and/or amplitude of incident light, serve to represent the holograms and to generate the complex-valued wave fronts of the scene. The refresh rate of the light modulator means must be sufficiently high in order to be able to represent a moving scene.
Because of the coherence of lasers, disturbing patterns, which are also known as speckle patterns or granulation, occur in the observer plane when using laser light for illuminating a light modulator. Speckle can be described as a granulation-like interference pattern which is created by interference of multiple light waves with statistically irregularly distributed phase differences. It disturbs the observer in watching the reconstruction of the scene, and it causes spatial noise there.
Speckle patterns can generally be reduced by temporal and/or spatial averaging of reconstructions of the scene in the observer eye. The observer eye always averages out multiple reconstructions with different speckle patterns presented to it, which results in a smoothing of the contours of the reconstructed scene.
According to document DE 195 41 071 A1, for example, a rotating glass plate is put into the optical path in order to temporally average the granulation or speckle patterns when checking the authenticity of a hologram. It rotates at a frequency which matches the frequency of a detector used for recording. Speckle patterns do thus not occur as disturbing effects.
However, such a method can only be applied for reducing two-dimensional, plane speckle patterns, where the diffusing plate must be disposed in the plane of the speckle patterns. The disadvantage of this method is that too much light is lost because of a diffusing plate in the light path. Further, it shall be avoided to use a mechanically rotating component in designing a holographic display device.
Another known method of reducing speckle patterns is to compute the scene with a given number of different random phases, and to represent the resultant holograms on a light modulator means one after another at a fast pace. However, the computational load increases substantially because of the many hologram computations. Further, a light modulator means provided to represent the holograms must have a very fast refresh rate.